Sensitometers are used for monitoring photographic and radiographic processes, and then making it possible to control these processes. However, in order to do this, it is necessary to identify parameters which reliably reflect the state of the process, and which can be conveniently measured on a regular basis.
In the case of a photographic process, it is common to illustrate the photographic response of a particular film, following development in that process, by a curve, sometimes called the "characteristic curve" for the material being measured. This characteristic curve represents the relationship between developed density-and the logarithm of exposure, and is often referred to as the H & D curve, after Hurter and Driffield, The Journal of the Society of Chemical Industry, No. 5, Vol. IX, 31st May 1890.
The "characteristic curve" is determined using a control strip as is well known in the art. The control strip is produced by taking a small piece of film and exposing it in a sensitometer by contact with an original step wedge, which has, typically, 21 densities in steps of 0.15 log exposure units (for X-ray films, for example), with light of a colour appropriate to the type of film being used for process control (typically either blue or green for X-ray films). The exposed strip is processed in the processor whose performance is being monitored, and is then ready to be measured.
Densities measured on the control strip are plotted against the relative log exposure, and important process control parameters from the resulting curve can be obtained which characterise the state of the process.